We propose to study a spontaneous polyarthritis of dogs which closely mimicks human rheumatoid arthritis in all parameters thus far investigated and which appears to be a unique experimental model. An ample supply of clinical material is assured. The clinico-pathologic, radiographic and immunologic features will be studied in depth, and a search made for possible initiating factors. This will include bacteria and mycoplasma cultures, and tissue culture and electron microscopic examination for virus. Experimental manipulations will be performed to investigate the role of immune complexes and complement in the disease process and a study of possible genetic factors involved in the canine disease will be undertaken. Many of these investigations could not be performed in man and the canine model offers very considerable investigative potential.